1. Field of the Invention
The present invention relates to a light emitting device with a light emitting element that emits fluorescent light or phosphorescent light upon application of electric field to a pair of electrodes of the element which sandwich a organic compound-containing layer (hereinafter, a light emitting layer comprising an organic material), and to a method of manufacturing the light emitting device. In this specification, the term light emitting device includes an image display device, a light emitting device and a light source (including illuminating device). Also, the following modules are included in the definition of the light emitting device: a module obtained by attaching to a light emitting device a connector such as an FPC (flexible printed circuit; terminal portion), a TAB (tape automated bonding) tape, or a TCP (tape carrier package); a module in which a printed wiring board is provided at an end of the TAB tape or the TCP; and a module in which an IC (integrated circuit) is directly mounted to a light emitting element by the COG (chip on glass) system.
2. Description of the Related Art
Light emitting elements, which employ organic compounds as light emitting member and are characterized by their thinness and light-weight, fast response, and direct current low voltage driving, are expected to develop into next-generation flat panel displays. Among display devices, ones having light emitting elements arranged to form a matrix shape are considered to be particularly superior to the conventional liquid crystal display devices for their wide viewing angle and excellent visibility.
It is said that light emitting elements emit light through the following mechanism: a voltage is applied between a pair of electrodes that sandwich a light emitting layer comprising an organic material, electrons injected from the cathode and holes injected from the anode are re-combined at the luminescent center of the light emitting layer comprising the organic material to form molecular excitons, and the molecular excitons return to the base state while releasing energy to cause the light emitting element to emit light. Known as excitation states are singlet excitation and triplet excitation, and it is considered that luminescence can be conducted through either one of those excitation states.
Such light emitting devices having light emitting elements arranged to form a matrix can employ passive matrix driving (simple matrix light emitting devices), active matrix driving (active matrix light emitting devices), or other driving methods. However, if the pixel density is increased, active matrix light emitting devices in which each pixel (or each dot) has a switch are considered as advantageous because they can be driven with low voltage.
Organic compounds for forming a layer containing an organic compound (strictly speaking, light emitting layer), which is the main part of a light emitting element, are classified into low molecular weight materials and polymeric (polymer) materials. Both types of materials are being studied but polymeric materials are the ones that are attracting attention because they are easier to handle and have higher heat resistance than low molecular weight materials.
The conventional active matrix type light emitting device has the structure comprising a light emitting element in which an electrode electrically connected with TFT on a substrate is formed as an anode, a light emitting layer comprising an organic material is formed thereon, and cathode is formed thereon. And light generated at the light emitting layer comprising the organic material can be observed at the TFT side through the anode that is a transparent electrode.
Therefore, manufactured in the present invention is an active matrix light emitting device that has a light emitting element with a structure called a top emission structure. In the top emission structure, a TFT side electrode which is electrically connected to a TFT on a substrate serves as an anode, a light emitting layer comprising an organic material is formed on the anode, and a cathode that is a transparent electrode is formed on the light emitting layer comprising the organic material. Or, an active matrix light emitting device that has a light emitting element with the structure in which the first electrode serves as a cathode, a light emitting layer comprising an organic material formed on the cathode, and an anode that is a transparent second electrode formed on the light emitting layer comprising the organic material is formed.
Not all of light generated in the light emitting layer comprising the organic material cannot be observed by observers through the transparent electrode serving as the cathode. For example, light emitted in the lateral direction (the direction parallel to the substrate face) is not taken out and therefore is a loss. An object of the present invention is to provide a light emitting device structured so as to increase the amount of light which is taken out in a certain direction after emitted from a light emitting element, as well as a method of manufacturing this light emitting device.